The invention relates to hot melt ink compositions to a method of improving the color strength of hot melt ink formulations.
Publication gravure uses solvent based inks almost exclusively. These inks, which dry by solvent evaporation, usually contain pigment, resins, volatile solvents like toluene or mineral spirits and additives such as waxes and silicones. Solvents are generally recovered for reuse. Government regulations and environmental concerns regarding the emissions of volatile organic compounds (VOCs) in the air are becoming more stringent. Therefore, it would be desirable to develop a zero VOC publication gravure ink system to satisfy these regulations.
One way to eliminate the solvents and other volatile organic compounds is to require that the ink solidify immediately on transfer from the cylinder to the paper. Some early patents (U.S. Pat, Nos. 2,147,651, 2,217,620, 2,264,315, 2,264,534 and 2,375,660) describe the use of a combination of amorphous resins and crystalline waxes or diluents for this purpose. Polyamides (U.S. Pat. No. 3,156,512) were used in combination with non-volatile glycol ethers and hydrocarbon oils as diluents. More recently, Siegwerk Druckfarben GmbH and Co. described a hot melt ink system based on acrylic resins (German Patent DE 42 05 636 A1; German Patent DE 42 05 713 A1 and U.S. Pat. No. 5,496,879) in a solid fatty acid diluent.
PCT patent application GB91/00812 describes hot melt ink formulations containing glycol monoesters with improves transparency and quality of color projection.
However, the hot melt ink formulations of the prior art still have relatively poor quality color strength and low optical print density.
The present invention relates to a hot melt ink formulation containing an alkylene diester, formed as a reaction product of a C4-C9 diol and at least one acid selected from the group consisting of a C16-C18 monofunctional acid and a C9-C10 difunctional acid.
The present invention also relates to a method of improving the color strength of a hot melt ink formulation by mixing an alkylene distearate into the formulation prior to applying the ink to a substrate, wherein said alkylene distearate is formed as a reaction product of a C4-C8 diol and at least one acid selected from the group consisting of a C16-C18 monofunctional acid and a C9-C10 difunctional acid.
The present invention also relates to a method of improving the optical print density of a hot melt ink formulation by mixing an alkylene distearate into the formulation prior to applying the ink to a substrate, wherein said alkylene distearate is formed as a reaction product of a C4-C8 diol and at least one acid selected from the group consisting of a C16-C18 monofunctional acid and a C9-C10 difunctional acid.
It has been surprisingly found that the objectives of the present invention could be realized by using an alkylene diester in a hot melt ink composition, wherein said alkylene diester is formed as a reaction product of a C4-C9 diol and at least one acid selected from the group consisting of a C16-C18 monofunctional acid and a C9-C10 difunctional acid.
Preferably, the monofunctional acid is cetyl acid, stearic acid or tall oil fatty acid and the difunctional acid is azelaic acid or sebacic acid.
Also preferably, the alkylene diester is a mixed diester of a carboxylic acid, a fatty acid and 1,6-hexandiol. More preferably, the alkylene diester is an alkylene distearate and the alkylene distearate is hexylene distearate.
It is also preferable that the hot melt ink composition of the present invention be a conventional hot melt formulation and include a resin, wax, dispersant, and color pigment.
The hot melt ink compositions of the present invention are suitable for use in ink jet, flexographic, intaglio, and gravure printing.
Preferably, the hot melt jet ink formulation of the present invention will include at least one rheology modifier comprising at least one plasticizer, to contribute to increased flexibility, cohesion, and durability of the ink composition. According to the invention, the most preferred jet ink plasticizer is an ester of benzoic acid present in an amount of no more than 25 wt. % preferably no more than 18 wt. %, most preferably 5 to 15 wt. % based on the weight of the ink composition.
The hot melt ink composition may additionally include rheology modifiers present in an amount of no more than 40 wt. %, preferably no more than 30 wt. %, most preferably no more than 20 wt. %, of the ink composition. These materials may also serve to improve scratch and rub resistance, make the ink more durable, and viscosity and tackiness.
Suitable rheology modifiers include stearic acid, lauric acid, linear polyethylene, phthalates, citrates, and oxidized polyethylenes, behenic acid, stearone, aluminum stearate, carnauba wax, microcrystalline waxes, paraffin wax, polyethylene wax, candelilla wax, montan wax, and the like.
Because hot melt jet ink compositions prepared in accordance with the invention are heated during jetting, antioxidants, to inhibit thermally induced oxidation, may be added to the composition. Suitable antioxidants include those conventionally used such as dibutyl hydroxy toluene compounds and the like. The antioxidant should be present in the amount of 0.1 to 5 wt. %, based on the weight preferably 0.5 to 3.0 wt. %, of the ink composition.
Suitable coloring agents, present in amount of at least 0.1 to 9 wt. %, preferably 0.5 to 3 wt. % based on the weight of the ink composition, include pigments and dyes. Any dye or pigment may be chosen provided that it is capable of being dispersed in the hot melt ink composition, is compatible with the other ink components, and is stable at a temperature of 100xc2x0 C. to 140xc2x0 C. As for use as a pigment, the pigment particles should have a diameter of less than 1 micron.
The hot melt jet ink compositions of the present invention are generally prepared in two steps. In the first step, the colorant dispersion is prepared by combining together all the dispersion ingredients except for the coloring agent, heating the mixture to its melting point, and slowly stirring the mixture until homogeneous. Next the coloring agent is added slowly and mixed to homogeneity. The dispersion is then collected and solidified at room temperature.
The second step consists of combining the ink composition ingredients except for the dispersion containing the coloring agent, heating the mixture to its melting point, which generally does not exceed 135xc2x0 C., and slowly stirring the mixture until homogeneous. The dispersion is slowly added to the mixture which is maintained in the molten state and the resultant molten mixture stirred until uniform. The molten mixture may then be filtered to remove particles larger than 1 micron in size when the coloring agent is a pigment.
The properties of the present hot melt inks should be similar to those of conventional inks. As with conventional gravure inks, print viscosity should be about 10 cps or less. As a hot melt ink, this viscosity must be attained at about 200xc2x0 F. to 275xc2x0 F. without producing any VOCs (or smoke) at this temperature.